The difference lies in the cell wall of the two types; gram-positive bacteria have a high amount of peptidoglycan in their cell wall which the stain interacts with, while gram-negative bacteria have a cell wall made primarily of lipopolysaccharide[?]. The gram-negative cell wall is similar to a cytoplasmic membrane[?], typically only a few layers thick and generally much thinner than gram-positive types.

Many species of gram-negative bacteria are pathogenic. This pathogenic capability is usually associated with certain components of their cell walls, particularly the lipopolysaccharide (endotoxin[?]) layer.